1. Field of the Invention
The present invention generally relates to a radio communication system for a mobile station and, more particularly, to a radio communication system between a vehicle moving on a road and a stationary station having an antenna located along the road.
2. Description of the Related Art
Conventionally, radio communication systems have been developed to provide communication between a mobile station provided on a vehicle moving on a road and a stationary station provided with an antenna installed along the road. For example, Japanese Laid-Open Patent Application No. 61-262326 discloses a radio communication system for a vehicle moving on a road. This radio communication system comprises a mobile station having a loop antenna provided on the vehicle and a stationary station having a loop antenna installed along the road. In this radio communication system, the axis of one of the loop antennas of the mobile station and the stationary station is parallel to the surface of the road, and the axis of the other of the loop antennas is perpendicular to the surface of the road. This arrangement of the loop antennas is determined so as to eliminate a dead zone formed between the fields formed by the loop antennas which is harmful to the radio communication.
In the conventional system, the loop antenna of the stationary station is located on one of the sides of the road. Thus, there is a problem in that a uniform field intensity cannot be obtained over the entire width of the road since the field intensity rapidly attenuates as a distance from the loop antenna of the stationary station is increased.
An AM radio antenna system provided in a highway tunnel comprises, as shown in FIG. 1, parallel antennas 10 and 11 which are provided on the ceiling of the tunnel. In this system, a uniform field intensity can be obtained along the moving direction of the mobile station or the longitudinal direction of the road. However, there is a problem in that the field intensity measured in a vertical direction or in a traverse direction greatly changes depending on a distance from the antennas 10 and 11 and a position relative to the antennas 10 and 11. Additionally, if an antenna of this type is constructed outside a tunnel, a construction of the antenna system becomes large since the antennas 10 and 11 must be located above the road. This results in an increase in a construction cost of the antenna system.
Another conventional antenna system is a leakage cable system used for, for example, a train railway as shown in FIG. 2. In FIG. 2, a leakage cable 14 runs along one of the sides of the pair of rails 12 and 13. In this system, there also be a problem in that a uniform field intensity cannot be obtained in a direction perpendicular to the longitudinal direction of the leakage cable 14 since a field intensity decreases as a distance from the leakage cable 14 is increased. Additionally, the leakage cable 14 is expensive.